Mass spectrometers are used in determining the concentration of gas components of a gas mixture. In order to achieve the high degree of accuracy and reliability that is required in determining the concentration of the gas components, it is important that the mass spectrometer be properly calibrated. Generally, there are two types of calibration--an initial or a full calibration and the calibration conducted during operation using a reference or calibration gas mixture. The initial or the full calibration is performed infrequently, for example, during manufacture and before shipment or when it is decided that a complete calibration is necessary for desired accuracy. In conjunction with the much more frequent calibration conducted by the user in order to reduce the times that a more time extensive full calibration is performed, significant amounts of calibration or reference gas are required for each calibration, with much of the calibration gas being wasted. In conducting a single calibration procedure, a typical range of calibration gas that is allowed to exit the tank containing the calibration gas is a minimum of about fifty cubic centimeters (cc) to several hundred cubic centimeters of the gas. Because only a very small volume of calibration gas is actually utilized in performing a proper calibration, much of the calibration gas is wasted. This loss of calibration gas is often times not acceptable because of the cost of the calibration gas. Relatedly, because of the amount of calibration gas that is used or lost for each calibration of a mass spectrometer, relatively infrequent calibrations of the mass spectrometer are made, e.g., instead of being implemented as frequently as every few minutes or even seconds, the calibration procedure is performed only after the passage of hours or even days from the previous calibration.
The importance of optimizing use of calibration gas is most evident when considering the cost of small volumes of calibration gas. In many instances, the cost of suitable calibration gas is in the range of hundreds of dollars for a few liters of calibration gas. The high cost of calibration gas becomes most apparent when considering a problem found in mass spectrometers used in determining and quantifying certain gases, such as mass spectrometers being used in the medical field, for example. For example, when oxygen is one of the component gases of an inputted sample gas mixture, the oxygen can chemically react with a carbonaceous film formed within the mass spectrometer, such as the carbonaceous film developed by a hot filament source in the ion source assembly. Such a reaction would produce carbon dioxide and carbon monoxide. Carbon monoxide tends to interfere with the obtaining of accurate measurement gas components, which are desirable to measure with the gas spectrometer. For example, the carbon monoxide would interfere with nitrogen gas determination because carbon monoxide and nitrogen have nearly the same mass. Such a problem could be avoided by providing an ion collection device that has an extremely high resolution; however, such a mass spectrometer would cost hundreds of thousands of dollars. It is known to use a calibration gas in order to measure the magnitude of the interfering gas and generate a correction factor, for example, in a laboratory setting where conditions are suitable for readily determining and making the necessary corrections. However, for on-line mass spectrometer instruments, this has not been practical. First, the presence of the carbon-based gas or gases varies with time. Consequently, even though a measurement might have been taken prior to using the instrument in order to correct for the presence of any carbon-based gas, such a correction would soon be inaccurate because the amount thereof would vary over time with the use of the mass spectrometer. Second, in prior art systems, even if it were considered to calibrate and determine a correction factor at a very frequent basis, on the order of seconds, the cost of the isotopic gas used in calibrating would be prohibitive. It is highly desirable to use an isotopic gas, such as isotopic oxygen because the isotopic oxygen combines with the carbon-based substance in such a manner that it will be easily collected and detected on an ion collection plate of the mass spectrometer. For example, the amount of carbon monoxide interfering or contaminating a nitrogen channel of the ion collection plate can be determined using a channel that is able to identify isotopic carbon monoxide. That is, the amount of isotopic carbon monoxide that is found during the calibration procedure is directly related to the amount of carbon monoxide that contaminates a nitrogen channel and which is developed when the sample gas includes oxygen and nitrogen. Even though the use of isotopic gas is of a significant benefit to the calibration procedure, its usage has been severely limited because of its considerable cost. Thus, it would be desirable to calibrate a mass spectrometer using an isotopic gas while minimizing the amount of gas required for each calibration procedure and, at the same time, increasing the frequency of calibration.
A further factor, which is important to the accuracy and precision of a mass spectrometer, relates to whether or not the mass spectrometer has a linear response when the ion source pressure varies over a range of pressures. That is, during operation of the mass spectrometer, the ion source pressure can vary and erroneous gas component determinations can be made if there is no linear response. Consequently, it is desirable during the calibration procedure to vary the ion source pressure in order to check the linearity of the response. This can be accomplished by controlling the opening of a valve communicating with the ion source. If the valve opening is controlled in incremental steps, during the calibration procedure, it can be determined whether or not the pass spectrometer has a linear response. If not, it can be adjusted. However, in prior art systems, such a procedure would result in utilizing and/or wasting relatively large amounts of calibration gas in order to check the mass spectrometer output at various ion source pressures by varying the valve opening in incremental steps. Thus, to improve mass spectrometer accuracy, it would be desirable to conduct the calibration process by checking the linearity of the system. Again, this is not feasible in prior art mass spectrometers because of the relatively longer time taken and amount of calibration gas required to perform the calibration.
Additionally, because of the amount of gas that exits a calibration gas tank during each calibration procedure, it is common practice to utilize a relatively large tank for housing calibration gas used with a mass spectrometer. For example, such a tank may contain about 2-3 liters of calibration gas at 1500 psi. Tanks of many times greater in size are also used. Because of such sizes, the calibration gas tank must be located outside of the mass spectrometer housing. Furthermore, even though it is of this relatively large size, it is often necessary to replace the calibration gas tank with another, filled tank or to re-fill the calibration gas tank after a few months of use.
Another drawback to known mass spectrometer calibrating systems is the requirement that a pressure regulator be used with the calibration gas tank. More specifically, the calibration gas is housed in a pressurized tank so that relatively more gas can be contained in the same size tank. In some instances where highly volatile gases must be present in the calibration tank, the upper limit for the pressure may be in the order of 50 psi. This limits the total volume of calibration gas that can be contained in the tank and cost/calibration is much greater as there is increased cost in preparing the tank containing the gases. Because it is critical that the calibration gas supplied to the mass spectrometer be at or near atmospheric pressure, the pressure regulator is used to control the pressure of the gas outputted from the prior art calibration gas tank. In practical applications, in addition to providing a pressure regulator to control the pressure of the calibration gas outputted from the tank, the supply line or tube leading from the tank to the mass spectrometer is exposed to atmospheric pressure. This set-up assures that the low-pressurized ionization and analyzing chambers are not subject to a high pressurized calibration gas. However, because of this set-up, additional calibration gas is wasted each time a calibration procedure is performed since dead space is created in the supply line.
Another factor influencing the accuracy and precision of the mass spectrometer analysis relates to the operation of electron multiplier devices, which act as amplifiers of electrons of the gas components that are received by the mass spectrometer at its ion collection plate. Prior art electron multipliers, whether multi-channel or single channel multipliers, do not take into account the fact that high level gases such as oxygen and nitrogen will develop a much greater signal strength than low level (tracer) gases. The difference in signal strength can be as much as a factor of 10.sup.6. In such a case, the high level gases would require little or no additional gain in the electron multiplier channel or channels while the low level gases would require a gain of about 10.sup.3. The present invention includes a method for changing the gain of the electron multiplier channel, or in the case of multiple channels, changing the gain of one or more channels of the multi-channel electron multiplier. However, a potential problem with this technique is that each channel of the electron multiplier could be operating on a different portion of its "aging curve" causing the relative gain between channels to change over time. The aging curve refers to the fact that the gain of the channel decreases as electric charge flows or ion current passes through the multiplier channel. That is, the gain of a channel of a microchannel plate, for example, decreases with the flow of cumulative electric charge through the channel. The aging is due to, for example, temperature changes, buildup of contaminants in the mass spectrometer, and the aging of the electron multiplier device itself. Consequently, to remedy this drift problem, while still providing selective signal gain for the different channels, it is important to calibrate frequently in order to determine any correction factor that would offset any adverse effect of individual channel gain change or amplifier drift.
With regard to previously issued patents which address calibration of mass spectrometers, U.S. Pat. No. 4,260,886 to Grilletto et al., issued Apr. 7, 1981, and entitled "Measurement of a Gas Constituent by a Mass Spectrometer" discloses that calibration can be achieved by using a reference sample that is about the same size as a test sample gas. U.S. Pat. No. 2,714,164 to Riggle et al., issued July 26, 1955, and entitled "Mass Spectrometer Sampling System" describes a mass spectrometer that includes a separate, calibrating channel, which may be the same in configuration as the channels for introducing unknown gas samples. U.S. Pat. No. 3,950,641 to Evans et al., issued Apr. 13, 1976, and entitled "Methods of Mass Spectrometry and Mass Spectrometers" discloses a two ion source mass spectrometer in which one ion source receives an unknown substance and the other ion source receives a known, reference substance whereby accurate chemical mass marking of the first mass spectrum generated using the unknown substance is provided using the second mass spectrum of the reference substance. None of the known mass spectrometers incorporates an automatic calibration feature.
Related to the present invention is the apparatus disclosed in U.S. Pat. No. 3,926,209 to Sodal et al., issued Dec. 16, 1975, and entitled "Method and Inlet Control System for Controlling a Gas Flow Sample to an Evacuated Chamber." This prior art apparatus describes a mass spectrometer that includes servo control for controlling the opening of a valve communicating with the ion source of a mass spectrometer for allowing a gas sample to pass into the mass spectrometer. Also related to the present invention is U.S. Pat. No. 4,560,871 to Bowman et al., issued Dec. 24, 1985, entitled "Actuator For Control Valves And Related Systems" and now assigned to the same assignee as the present invention. This patent discloses a valve for controlling the flow of an unknown gas into an ionization chamber of a mass spectrometer.
The calibration of apparatuses, other than mass spectrometers, is also well-known. U.S. Pat. No. 4,151,738 to Hyer et al., issued May 1, 1979, and entitled "Toxic Gas Monitor Having Automatic Calibration" discloses a system for monitoring for the presence of a toxic gas. The monitor is automatically calibrated by exposing a sensor to an atmosphere having a known concentration of the monitored constituent. In accomplishing the automatic calibration, atmospheric samples are measured periodically under the control of a sequencer and, at predetermined intervals, the sequencer initiates a recalibration cycle. In contrast to the present invention hereinafter described, such a system does not include a mass spectrometer in which a high degree of accuracy is required to sense and discriminate between a number of gases of a gas mixture. Importantly also, such a system apparently does not utilize expensive calibration gases and is, therefore, not concerned about wasting calibration gas.